Electrostatically actuated micro-electrical-mechanical system (MEMS) devices have been proposed for a variety of applications. One promising use for MEMS devices is in optical switching and steering devices. In such devices, movable micromachined mirrors are used as switching elements to direct input optical signals to desired outputs. The movement of the micromachined mirrors is accomplished by electrostatic actuation.
In a typical MEMS device, an individual mirror is affixed to a movable support structure (i.e., a gimbal) via torsional elements such as springs. The gimbal may be coupled to a frame, also via torsional elements. Typically, two torsional elements positioned on opposing sides of the mirror, couple the mirror to the gimbal, and define an axis for mirror rotation. Similarly, two torsional elements positioned on opposing sides of the gimbal, couple the gimbal to the frame, and may define an axis for gimbal rotation.
In a conventional device, electrodes are positioned under the mirror and gimbal. The electrodes are configured to rotate the mirror or gimbal in either direction about its axis. The mirror or gimbal rotates under the electrostatic force between the mirror and gimbal, and is balanced in equilibrium by the restoring force of the torsional elements. The degree of rotation depends upon the amount of voltage applied to the electrodes. Traditionally, a degree of rotation up to about 9 degrees is achievable. However, with the above-mentioned designs, a voltage of greater than about 150 volts may be required.
One disadvantage with the traditional switching element disclosed above is that large gaps between the mirror and the electrodes are needed for large-angle tilting. Because the electrodes are positioned under the mirror for operation, the electrodes must be positioned having a gap large enough that a desired tilt angle may be achieved. Additionally, the larger the mirror used, the larger the gap required. As a result of the varying tilt angles and resulting mirror placement, the device has to be made up of separate mirror and electrode components, which may be assembled manually. This is typically a tedious and expensive process and introduces additional room for error to occur. More importantly, because of the distance needed between the mirror and the electrodes for large-angle tilting, a higher actuation voltage may be required to operate the MEMS devices. This higher voltage requirement adversely affects the controllability of the device and, of course, increases power requirements of the device that is intended to function on as low a voltage as possible.
Another disadvantage with the traditional switching element disclosed above is that the electrodes are positioned within a path of motion of the mirror. In such situations it is possible for the mirror to physically contact the electrodes, possibly causing the lifetime of the device to diminish.